Geophysical prospectors have found seismic vibrators to be useful signal sources for imaging the earth. Conventional seismic acquisition in the past generally employed multiple vibrators acting together and initiated simultaneously to form a source array. In land-based operations, the vibrators are positioned at a source location and synchronized to the same pilot sweep signal. Once activated, the vibrators generate a sweep that typically lasts between five and twenty seconds and typically spans a predetermined range of frequencies. A recording system that is connected to a plurality of receivers, typically geophones for land-based seismic exploration, is employed to receive and record the response data. For reflection seismology, the record length is typically set to equal the sweep length plus a listen time equal to the two-way travel time, which is the time required for the seismic energy to propagate from the source through the earth to the deepest reflector of interest and back to the receiver. In typical applications the recorded seismic data is cross-correlated with the pilot sweep signal. If ambient noise is problematic, the sweep and recording process is repeated several times to form a stack, which is a method of averaging the recordings together to improve signal-to-noise ratio. Stacking can be performed on either the raw uncorrelated data or the correlated records. The vibrators are then moved to a new source location and the process is repeated.
The conventional method has a number of shortcomings some of which include: 1) poor source control; 2) harmonic distortion emissions by the vibrators; 3) intra-array statics because the vibrators are at different elevations or variations in the near surface that can affect source coupling to the earth; 4) spatial resolution issues due to array effects and limitation in source effort because of economic constraints; 5) control and synchronization problems associated with the use of multiple sources; and 6) mixed-phase data produced by the correlation process. Improvements in technology and reductions in the per channel cost Of recording have resulted in an industry push toward using point source-point receiver methods to overcome some of the problems associated with source arrays and large receiver arrays. In land surveys today, the use of point receivers has rapidly increased productivity in deploying the receiver spread. As a result, the vibrators have become the weak link in achieving efficient field operations.
Over the years a number of methods have been introduced to address shortcomings with conventional seismic survey methods. These new methods can generally be categorized as: 1) methods to improve data quality, and 2) methods to reduce cost or improve data acquisition efficiency. One method, titled “Method for Continuous Sweeping and Separation of Multiple Seismic Vibrators,” by Krohn and Johnson attempts to address both the data quality and data acquisition issues. (See WO/2005/019865). This method is an extension of the High Fidelity Vibratory System (“HFVS”) originally developed by MOBIL and ARCO in the late 1990's (See U.S. Pat. Nos. 5,719,821 and 5,721,710). The MOBIL-ARCO alliance developed a data acquisition and data processing technique that eliminates vibrator intra-array statics problems, mitigates vibrator control errors, provides minimum phase data, and provides high spatial resolution. However, in order to provide a cost effective method for effectively collecting point source data, a means to separate vibrators sweeping simultaneously was necessary.
HFVS, in its original implementation, required the collection of uncorrelated data sets. Measured source signals from each vibrator and received signals are required. For sweeps utilizing swept sine waves, HFVS requires a phase offset encoding scheme that is unique for each vibrator. To compensate for possible corruption of one of the sweeps, additional sweeps could be executed to create an over-determined data set useful for further improving the signal-to-noise ratio. Further, conventional HFVS technology requires, at a minimum, as many records as sources per source location to achieve source separation. For example, using four vibrators requires at least four records. With a 12 second sweep and a five-second listen for each record, the total duration is 68 seconds. Many recording systems may also require additional time for a system reset after each record. If, for example, the system reset time is two seconds the total duration increases to 76 seconds.
Continuous HFVS is a new EXXON-MOBIL technique that combines a variation of the EXXON-MOBIL Cascaded Sweep technique and HFVS. By linking sweeps together with no listen time between segments, recording acquisition time is reduced. For example, a four vibrator implementation with four 12 second sweep segments and a five-second listen time and collected in one record has a total duration of 53 seconds. Compared to the 68-second duration in HFVS, this approach can result in a 22 percent timesaving. In practice, the record is processed by dividing the longer record into shorter records, and then conventional HFVS processing is performed.
HFVS-methods have some technical drawbacks including: 1) low frequency noise in the inverted records because of an absence of low frequencies in the source signal; 2) not capturing all harmonic energy produced in the measured source signal; 3) a large data volume; 4) poor quality control because uncorrelated data is used (“shooting blind”); and 5) source outputs using phase offset encoding, resulting in highly correlated source signals that require reliable phase encoding for good separation.
With the Continuous HFVS method, there is a high potential for problems associated with crosstalk between segments. In conventional seismic acquisition, up-sweeps are used to mitigate the interference caused by harmonic distortion emissions. The correlation process moves artifacts associated with harmonic distortion into negative time so that they do not interfere with weaker reflections arriving later in time. Because phase-encoded cascaded up-sweeps repeat the same low frequencies during the listen time of a prior sweep segment and harmonic emissions are often strong during the low frequency generation portion of a sweep, harmonic emissions from a subsequent segment might arrive from a shallow reflector at the same time as weak deep reflection data due to emissions in an earlier sweep segment. This can happen even though correlation is not employed for several reasons. First, the process of inversion, which is the process of forming a ratio between a received signal and a measured source signal in the frequency domain, tends to deal with the phase between two signals in the same way. That is, the phase spectrum of a wavelet produced by the process of inversion and cross-correlation between the same two signals is the same. If the vibrator-measured signal for an upsweep does not fully characterize the harmonic emissions from the source, any residual harmonics will be mapped into negative time. This is similar to conventional seismic prospecting where a pilot signal that contains no harmonics is correlated with the seismic response data containing harmonic emissions. Second, the source-to-earth coupling problem is generally nonlinear. While the measured source signal is helpful, it does not completely characterize or measure all the radiated harmonic energy. Third, the source-to-earth coupling may change as the vibrator compacts the earth during the sweep or sweep segments. This compacting may cause the travel time from the vibrator to the receiver to deviate slightly from sweep to sweep or from sweep segment to sweep segment. Because phase encoding methods assume that the transmission path will remain constant between sweeps or sweep segments, data separation may be compromised. Fourth, with Continuous HFVS there is an increase in the potential for problems associated with this residual harmonic energy from a later sweep segment interfering with deep reflection events from an earlier segment. However, with standard FIFVS the consequences of some residual harmonic energy bleeding through are generally minimal. On an up-sweep, this residual harmonic energy will be mapped to the early part of the record where there are strong reflectors and not interfere with weak deep reflection events later in the record. Finally, for Continuous HFVS the assumption that sweep segment length exceeds the two-way travel time to the deepest reflector of interest does not guarantee residual harmonic energy will not cause interferences between sweep segments. While superposition and linearity assumptions may work with model data, these assumptions fail to simulate the real impact of residual harmonic energy. Further, the accepted industry guideline for crosstalk between the separated source signals to fall below −40 dB is demanding.
The cost of seismic surveys depends heavily on the time required to collect the data. To reduce the acquisition time a number of methods have been devised over the years. Methods for source separation disclosed vibrator sources that are operated concurrently to reduce the time required for acquiring seismic survey data. For example, two groups of vibrators shooting into the same receiver spread at different offsets can be used to form a composite record. Most of those methods involve some form of swept sine wave source signal and rely on properties of the sweeps to be separated by correlation. Some methods rearrange portions of a conventional swept sine wave to mitigate crosstalk between surveys due to cross-correlation between the sweeps employed (See U.S. Pat. Nos. 4,168,485 and 4,982,374). Others achieve separation by using phase encoding schemes sometimes combined with up-sweeps and down-sweeps (See U.S. Pat. No. 4,823,326), and still others use time delays (See U.S. Pat. No. 4,953,657). Still others use techniques such as slip-sweep that combine conventional swept sine waves, time delayed starts, and processing methods of F-T filtering, deconvolution, and migration to achieve separation (See WO 2006/018728).
Although fewer in number, there are several schemes for seismic prospecting that employ coded seismic signals generated by pseudorandom sequences (See U.S. Pat. Nos. 4,969,129, 6,704,245, and 6,807,508). Both U.S. Pat. Nos. 6,704,245 and 6,807,508 disclose the use of Pseudorandom Binary Sequences (“PRBS”) that are used to phase modulate a periodic sine wave and may include the phase modulation of a DC signal. PRBS of maximal length, also known as Galois sequences, can be generated from irreducible, or primitive, polynomials. These PRBS have the property that correlation side lobes are distributed fairly evenly and any artifacts produced by harmonic distortion noise emissions tend to be distributed throughout the record after correlation because all frequencies are generally emitted at all times. There is no temporal separation of spectral content as might occur in swept sine wave type sweeps. Additionally, spreading out the spectral content of the sweep, rather than being concentrated for only a portion of the sweep, reduces the temporal peak energy of a given spectral element. Thus structural resonances are not excited and windows on nearby structures are not rattled as badly as with swept sine waves. A large collection of sequences known as Gold codes can be generated from special pairs of irreducible polynomials of the same order called preferred pairs. These Gold codes have very desirable characteristics, one of which includes low cross-correlations among the sequences.
To fully realize the low autocorrelation side lobes and desirable cross-correlation properties between different weakly correlated PRBS sweeps, the measured signal is correlated with a repeated version of the pilot sweep made up of at least three sweep repetitions. The low side lobe and crosstalk between sweeps are compromised when the weakly correlated PRBS sweeps are band limited, a sweep sequence comprised of less than a full 2n−1 phase shifts is employed, or harmonic distortion is introduced. (The variable “n” is the sequence order. For example, for a polynomial order of 14 (n equals 14) the sequence length would be 16,383.) For a Gold code sequence, it has been shown that the members have cross-correlation and autocorrelation side lobes, θ(r), bounded by |θ(r)|≦2(n+1)/2+1 for n odd, and by |θ(r)|≦2(n+2)/2−1 for n even (See, eg. Dixon, Robert C., SPREAD SPECTRUM COMMUNICATIONS (1994)). Again for the example of n equal to 14, the maximum cross-correlation value would be 255 out of a zero lag autocorrelation value of 16,383 or 20 log (255/16383)=−36 dB of crosstalk for two sources working simultaneously. Generally this level of crosstalk is marginal at best and above the industry-acceptable level of crosstalk.
As has been mentioned before, vibrators produce harmonic distortion energy. For HFVS, less than all of the harmonic energy may be mapped into signal. With swept sine wave sweeps, the harmonic energy is well organized. However, using up-sweeps, the harmonic energy is pushed into negative time and is generally not a problem because it is masked by strong arrivals at early times. For pseudorandom sweeps, harmonic energy is not well organized, but is dispersed across the record. For a correlated record, the harmonic energy noise will occur in both positive and negative time and tends to be evenly distributed throughout the record at a low level that is potentially less likely to mask reflectors. Results obtained using pseudorandom sweeps should not be sensitive to where the harmonics are produced and measured because the correlation disperses the harmonics regardless where they enter the process. Further, the overall averaging effect makes sources and source coupling behave more like linear systems.
Another potential advantage to weakly correlated PRBS sweeps is that low frequency energy can be increased without running into vibrator stroke and pump limitations. A typical vibrator, such as the Nomad model 65 vibrator, available from Sercel, Inc., Houston, Tex., that is rated to produce about 62,000 pounds force, can only generate about 12,500 pounds peak force at 3 Hz with a swept sine wave input. At that frequency and output level, the mass is moving about 3 inches peak-to-peak and in danger of hitting the stops. However, using a PRBS sweep, that same 3 Hz energy can be spread out over the entire sweep length at a lower temporal peak-value, producing higher levels of low frequency energy if desired. With regard to the hydraulic power pack and pump, a swept sine wave requires the hydraulic power pack to deliver peak flow at the low frequencies. The peak flow can exceed 200 gallons per minute (“gpm”) at the low end of the sweep. Most vibrators cannot deliver this high demand over an extended length of time and, as a result, the hydraulic system pressure drops well below its nominal differential supply pressure of 2,900 psi. Lower system pressure results in lower force generation capability and higher distortion. Typically, flow demand drops to its lowest point between 20 Hz to 50 Hz, where it may go down to as low as 10 gpm to 20 gpm and then begins to increase at the higher frequencies. At the higher frequencies, the combined effect of fluid compressibility and fluid inertia create inefficiency in the system that requires higher flow to sustain a high force output from the vibrator. However, with the PRBS sweep, the peak demand from the hydraulic power pack and pump should be reduced substantially. This is because PRBS sweeps generate all frequencies concurrently. Thus, the peak flow should approach the average flow seen over the course of a conventional chirp sweep and peak demand will likely fall to approximately 100 gpm.